The present invention relates to load tap changers and, more particularly, to selector switch assembly for a load tap changer.
As is well known, a transformer converts electricity at one voltage to electricity at another voltage, either of higher or lower value. A transformer achieves this voltage conversion using a primary winding and a secondary winding, each of which are wound on a ferromagnetic core and comprises a number of turns of an electrical conductor. The primary winding is connected to a source of voltage and the secondary winding is connected to a load. Voltage present on the primary winding is induced on the secondary winding by a magnetic flux passing through the core. The voltages induced on each turn of the secondary winding are cumulative and therefore the voltage output from the secondary winding is proportional to the strength of the magnetic flux and the number of turns in the secondary winding. Since the amount of magnetic flux generated by the primary winding is proportional to the number of turns in the primary winding and the voltage produced by the secondary winding is proportional to the magnetic flux surrounding the secondary winding, the output voltage of the transformer is generally equal to the input voltage times the ratio of the number of turns in the secondary winding over the number of turns in the primary winding. Thus, by changing the ratio of secondary turns to primary turns, the ratio of output to input voltage can be changed, thereby controlling or regulating the output voltage of the transformer. This ratio can be changed by effectively changing the number of turns in the primary winding and/or the number of turns in the secondary winding. This is accomplished by making connections between different connection points or “taps” within the winding(s). A device that can make such selective connections to the taps is referred to as a “tap changer”.
Generally, there are two types of tap changers: on-load tap changers and de-energized or “off-load” tap changers. An off-load tap changer uses a circuit breaker to isolate a transformer from a voltage source and then switches from one tap to another. An on-load tap changer (or simply “load tap changer”) switches the connection between taps while the transformer is connected to the voltage source. A load tap changer may include, for each phase winding, a selector switch assembly, a bypass switch module and a vacuum interrupter module. The selector switch assembly makes connections between taps, while the bypass switch module connects the tap(s) to a main power circuit. During tap changes, the vacuum interrupter module safely carries the current between the tap(s) and the main power circuit. A drive system moves the selector switch assembly, the bypass switch module and the vacuum interrupter module. The operation of the selector switch assembly, the bypass switch module and the vacuum interrupter module are interdependent and carefully choreographed. As such, these assemblies and, load tap changers in general, are conventionally complex devices that are difficult to manufacture and must be carefully maintained. Moreover, conventional tap changers are based on old configurations that are heavily dependent on mechanical interconnections.
Thus, there is a need to provide an improved selector switch assembly for a load tap changer that has a robust configuration, is less expensive, and easier to manufacture than conventional configurations.